Either you are suffering from a vitamin deficiency in your diet, or you may have gum disease. Your dentist will advise you which it is, and the measures you can take to clear it up.
I had this once. Its called gum disease. It happens because food gets in between your teeth and normal brushing doesn't get rid of it. Use dental floss to clean between your teeth on a daily basis. If its bad like mine was you'll need to go to a good dentist first. And I do emphasis a GOOD dentist, it was because of the slackness of dentists I went to that mine was so advanced in the first place. Good Luck!!